Heat exchanger with brazed plates

ABSTRACT

The invention concerns a heat exchanger comprising a stack of plates defining passages, containing corrugated fins comprising a transverse section with repeated corrugated pattern extending between two upper and lower end planes. The pattern comprises a base corrugated pattern (M) comprising corrugated legs ( 13 ) linked to corrugated summits ( 16 ) and corrugated bases ( 17 ), said base pattern being modified by a sub-pattern (M 1 ) which defines, between at least some corrugated legs, additional leading edges ( 20, 21 ) located at an intermediate level between the planes. The invention is applicable to cryogenic gas-gas heat exchangers.

[0001] The present invention relates to a brazed-plate heat exchanger,whose passages contain at least one corrugated fin of the typecomprising, in cross section, a repeated corrugated pattern whichextends between two upper and lower extreme planes defined by the platesof the exchanger.

[0002] The invention is in particular applicable to gas-gas cryogenicexchangers for air distillation apparatuses, such as the main heatexchange line of these apparatuses, which cools the incoming air byindirect heat exchange with the cold products from the distillationcolumn.

[0003] The corrugated fins in question are widely used in brazed-plateheat exchangers, which have the advantage of offering a large heatexchange surface area in a relatively small volume, and of being easy tomanufacture. In these exchangers, the fluid flows may be cocurrent,countercurrent or crosscurrent flows.

[0004]FIG. 1 of the appended drawings shows, in perspective, withpartial cutaways, an example of such a heat exchanger, of conventionalstructure, to which the invention is applicable. In particular, it mayinvolve a cryogenic heat exchanger.

[0005] The heat exchanger 1 shown consists of a stack of parallelrectangular plates 2 which are all identical and which between themdefine a plurality of passages for fluids to be brought into indirectheat exchange relationships. In the example shown, these passages are,in succession and cyclically, passages 3 for a first fluid, 4 for asecond fluid and 5 for a third fluid.

[0006] Each passage 3 to 5 is bordered by closure bars 6 which definethe passage, leaving inlet/outlet windows 7 of the corresponding fluidfree. Placed in each passage are spacer waves or corrugated fins 8acting both as thermal fins, as spacers between the plates, especiallyduring brazing and in order to avoid any deformation of the plates whenusing pressurized fluids, and for guiding the fluid flows.

[0007] The stack of plates, closure bars and spacer waves is generallymade of aluminum or aluminum alloy and is assembled in a singleoperation by furnace brazing.

[0008] Fluid inlet/outlet boxes 9, of semicylindrical overall shape, arethen welded to the exchanger body thus produced so as to sit over therows of corresponding inlet/outlet windows, these boxes being connectedto fluid feed and discharge pipes 10.

[0009] There are various types of spacer waves 8. Thus mention may bemade of straight fins, with rectilinear, possibly perforated,generatrices, fins known as “herringbone” fins, with sinuousgeneratrices, louvered fins, the wave legs of which have rows ofrecesses, and partially offset or “serrated” fins.

[0010] In these various fins, the wave may have a square, rectangular,triangular, sinusoidal, etc., cross section.

[0011] The aim of the invention is to improve the thermal performance ofexchanges with corrugated fins. To this end, the subject of theinvention is a brazed-plate heat exchanger, of the type comprising astack of parallel plates which define a plurality of generallyflat-shaped fluid flow passages, closure bars which define thesepassages, and corrugated fins placed in the passages, at least some ofthe corrugated fins being of the type comprising, in cross section, arepeated corrugated pattern extending between two upper and lowerextreme planes defined by two adjacent plates of the exchanger,characterized in that the pattern comprises a basic corrugated patterncomprising wave legs connected by wave crests and wave troughs, thisbasic pattern being modified by a subpattern which defines, between atleast some pairs of wave legs, additional exchange surfaces located atan intermediate level between the two extreme planes.

[0012] According to other optional aspects:

[0013] the subpattern defines a subcorrugation which extends only over aportion of the distance which separates the two extreme planes.

[0014] the subpattern comprises at least one nonvertical part located atan intermediate level between the two extreme planes.

[0015] the subpattern further comprises pairs of limbs which connect thenonvertical parts alternately to a wave crest and to a wave trough.

[0016] the limbs are vertical.

[0017] the subpattern comprises at least one additional oblique exchangesurface.

[0018] the subpattern has a V-shaped section.

[0019] the subpattern comprises a step adjacent to at least some legs ofthe main pattern.

[0020] the fin is partially offset.

[0021] the offset distances ensure that the main pattern is offset bothwith respect to itself and with respect to the subpattern.

[0022] the pattern repeats every N rows of waves, where N ≧3 and inparticular, N=4.

[0023] at least some parts of at least some troughs and/or subpatternscomprise a notch in at least one leading and/or trailing edge and in atleast part of their height or their width.

[0024] the wave has a square, rectangular, triangular or sinusoidalcross section.

[0025] the basic corrugated pattern is constant over the entire lengthof the two extreme planes.

[0026] The following will mainly concern serrated fins, but it will beunderstood that the invention is also applicable to other types of finsdescribed above.

[0027] Exemplary embodiments of the invention will now be described withrespect to the appended drawings, in which:

[0028]FIG. 2 shows, in perspective, a serrated fin according to theinvention;

[0029]FIG. 3 is an end view of this fin;

[0030]FIG. 4 is an end view of a variant;

[0031]FIG. 5 shows, in perspective, another serrated fin according tothe invention;

[0032]FIG. 6 is a view in exploded perspective of the fin of FIG. 5;

[0033]FIG. 7 is an end view of the fin of FIG. 5; and

[0034]FIG. 8 is an end view of another serrated fin according to theinvention.

[0035] The serrated fin 1 shown in FIGS. 2 and 3 has an overall maincorrugation direction D1 and comprises a large number of adjacent waverows 12A, 12B, . . . , which are all identical and are oriented in adirection D2 perpendicular to the direction D1.

[0036] For convenience in the description, it will be assumed that, asshown in FIG. 2, the directions D1 and D2 are horizontal, similarly withthe plates 2 of the exchanger.

[0037] Each wave row 12 has, in cross section perpendicular to D1, abasic pattern M which comprises two vertical wave legs 13. With respectto an overall sense F of the flow of the fluid along the direction D1 inthe passage in question, each leg comprises a leading edge 14 and atrailing edge 15. The legs are alternately connected along their upperedge by means of a rectangular, flat and horizontal wave crest 16, andalong their lower edge by means of a wave trough 17 which is alsorectangular, flat and horizontal.

[0038] The basic pattern M is modified by a subpattern M1 consisting ofa rectangular projection extending downward in the middle of each crest16 and upward in the middle of each trough 17.

[0039] Each subpattern M1 consists of one flat end part 18 located halfway between the extreme planes defined by the adjacent plates 2, and twovertical limbs 19 which connect the edges thereof to the correspondingcrest 16 or trough 17.

[0040] Thus, each subpattern forms a notch which comes in between thetwo adjacent legs 13. This notch defines three additional exchangesurfaces, that is a horizontal exchange surface 20 and two verticalexchange surfaces 21.

[0041] The rows 12 are offset one with respect to another in thedirection D2, alternately in one sense and in the other. By using theterm “pitch” to refer to the distance p which separates two successivelegs 12 (ignoring the thickness e of the thin sheet material forming thewave), the offset is alternately p/6 in one sense and in the other,while the notch width M1 is p/3.

[0042] Thus, each row 12 is connected to the following row 12 by meansof the crests 16, along right-handed segments 22 of length p/6, and bymeans of the troughs 17, along right-handed segments 23 of the samelength p/6. The offset planes are the vertical planes such as P_(AB) andthe offset lines, seen from the top, are denoted by 24.

[0043] Moreover, l is used to denote the length of each row 12 in thedirection D1, this length being called the “serration length”, and h isused to denote the height of the fin.

[0044] In practice, the shapes of various wave parts may differ to agreater or lesser degree from the theoretical shapes described above,especially with regard to the flatness and the rectangular shape of thefacets 13 and 16 to 19, and the verticality of the facets 13 and 19.

[0045] Seen from the end (FIG. 3), the patterns M are offset sidewayswith respect to themselves and with respect to the patterns M1, that isto say that the legs 13 of a given serration row 12 each appear betweena leg 13 of the adjacent rows and a limb 19 of a neighboring subpatternM1. Conversely, the limbs 19 of the same row 12 each appear eitherbetween two limbs 19, or between a limb 19 and a leg 13, of the adjacentrows 12.

[0046] Because of the presence of the subpatterns M1, the flowseparation is increased at each offset line 24, which increases thetemperature difference between the fluid and the fin, thus increasingthe heat flux exchanged. The presence of additional leading edges 20 and21 further generates turbulence within the fluid, which promotes heattransfer by convection toward the core of the flow and not by conductionthrough the limiting layer, which promotes heat exchange.

[0047] The variant of FIG. 4 differs from that of FIG. 3 by a greaterdepth of the notches M1, this depth changing from about h/2 to 2h/3. Inthis way, the preferential flow regions, which miss out on thebeneficial effect of the notches M1 described above, are reduced.

[0048] With the same objective, FIGS. 5 to 7 show a serrated fin whosepattern M+M1 repeats not every other row, but one row in N, where N≧3.This makes it possible to increase the symmetry of flow. In the exampleshown, N=4. Four successive rows 12A to 12D will subsequently bedescribed below.

[0049] As previously, each row has the same rectangular basic pattern M,comprising vertical legs 13 spaced apart by the pitch p and alternatelyconnected by a wave crest 16 of width p and by a wave trough 17 of thesame width p. The pattern M is modified by a subpattern M1A to M1D:

[0050] subpattern M1A: in each upwardly open corrugation, the lower partof the right leg 13 is deformed by a step which comprises a horizontalpart 24 located half way up the leg and a vertical part 25 located halfway between this leg and the other leg of the corrugation. Thus, thelower half of the leg and the right half of the adjacent wave trough areremoved, as shown by chain line;

[0051] subpattern M1B: in each downwardly open corrugation, the upperpart of the left leg 13 is deformed by a similar step, that is to say arectangular step of dimensions p/2 and h/2;

[0052] subpattern M1C: in each upwardly open corrugation, the lower partof the left leg 13 is deformed by a similar step. This subpattern istherefore symmetrical with respect to the subpattern M1A;

[0053] subpattern M1D: in each downwardly open corrugation, the upperpart of the right leg 13 is deformed by a similar step. This subpatternis therefore symmetrical with respect to the subpattern M1B;

[0054] Moreover, in this embodiment, the offset from one row to the nextis p/2, alternating in one sense and in the other (?). FIGS. 5 and 6indicate two neighboring vertical planes P1 and P2, in order to make iteasier to understand the structure of the fin.

[0055] The embodiment of FIG. 8 is derived from that of FIG. 3 in thateach subpattern M1 is triangular and is no longer rectangular or square.Thus two oblique leading edges 25, which are symmetrical with respect tothe vertical plane of symmetry P of the wave, are inserted into eachwave.

[0056] In the example shown, the height of the triangle is h/2, but, asbefore, it may have a different value, especially a value greater thanh/2 in order to reduce the preferential flow regions.

[0057] In all the above examples, high thermal performance of theexchanger, with highly divided and turbulent flow and with atwo-dimensional, or even three-dimensional configuration is obtained.

[0058] Note that the fins may be manufactured by simple folding of aflat product on a press or using a cogged wheel, as for the conventionalcorrugated, especially serrated, fins. This is because the surfaces areall developable, such that it is enough to match the profile of thefolding tools.

[0059] The presence of the subpatterns M1 causes passage restriction atthe offset lines, and therefore pressure drops. These pressure drops canpossibly be reduced by providing notches carefully placed in at leastsome leading and/or trailing edges of the patterns M and/or M1. Thesenotches will preferably be located facing the leading and/or trailingedges of the subpatterns M1, or therewithin, as indicated in chain lineby 26 in FIG. 2.

[0060] Whatever the fin type, the latter may be made either from solidsheet metal, or from perforated sheet metal or sheet metal providedotherwise with apertures.

1. A brazed-plate heat exchanger, of the type comprising a stack ofparallel plates (2) which define a plurality of generally flat-shapedfluid flow passages (3 to 5), closure bars (6) which define thesepassages, and corrugated fins (8) placed in the passages, at least someof the corrugated fins (8) being of the type comprising, in crosssection, a repeated corrugated pattern extending between two upper andlower extreme planes defined by two adjacent plates of the exchanger,characterized in that the pattern comprises a basic corrugated pattern(M) comprising wave legs (13) connected by wave crests (16) and wavetroughs (17), this basic pattern being modified by a subpattern (M1)which defines, between at least some pairs of wave legs, additionalexchange surfaces (20, 21) located at an intermediate level between thetwo extreme planes.
 2. The exchanger as claimed in claim 1,characterized in that the subpattern (M1) defines a subcorrugation whichextends only over a portion of the distance which separates the twoextreme planes.
 3. The exchanger as claimed in claim 1 or 2,characterized in that the subpattern comprises at least one nonverticalpart (18) located at an intermediate level between the two extremeplanes.
 4. The exchanger as claimed in claim 3, characterized in thatthe subpattern (Ml) further comprises pairs of limbs (19) which connectthe nonvertical parts (18) alternately to a wave crest (16) and to awave trough (17):
 5. The exchanger as claimed in claim 4, characterizedin that the limbs (19) are vertical.
 6. The exchanger as claimed inclaim 1 or 2, characterized in that the subpattern (Ml) comprises atleast one additional oblique exchange surface (25).
 7. The exchanger asclaimed in claim 6, characterized in that the subpattern (M1) has aV-shaped section.
 8. The exchanger as claimed in claim 1 or 2,characterized in that the subpattern (M1) comprises a step (24, 25)adjacent to at least some legs (13) of the main pattern (M).
 9. Theexchanger as claimed in any one of claims 1 to 8, characterized in thatthe fin (11) is partially offset.
 10. The exchanger as claimed in claim9, characterized in that the offset distances ensure that the mainpattern (M) is offset both with respect to itself and with respect tothe subpattern (M1).
 11. The exchanger as claimed in claim 10,characterized in that the pattern (M, M1) repeats every N rows of waves,where N≧3.
 12. The exchanger as claimed in claim 11, characterized inthat N=4. The exchanger as claimed in any one of claims 1 to 12,characterized in that at least some parts of at least some troughs (M)and/or subpatterns (M1) comprise a notch (26) in at least one leadingand/or trailing edge and in at least part of their height or theirwidth.